The present invention relates to an organic electroluminescent device (hereinafter referred to as an organic EL device). More precisely, it relates to an organic EL device which produces high luminance and is durable and which can be fabricated with ease.
Organic EL devices in which the light-emitting layer is of a mixture of a hole-transporting compound and an electron-transporting compound are disclosed in Japanese Patent Laid-Open No. 250292/1990, 291696/1990 or 790/1991. In the devices disclosed, however, the organic compounds used are poor in hole-transporting or electron-transporting capabilities, and the devices with them could hardly achieve high luminance. In addition, the compounds are poor in heat resistance and durability. Therefore, another problem with the devices is that, when they are driven continuously, the compounds therein are readily crystallized with the result that the luminance of the devices is often lowered. Moreover, it is not easy to uniformly mix the hole-transporting compound and the electron-transporting compound for the devices. Therefore, still another problem with the devices is that the light from them is often not uniform and that the organic compounds for them could hardly formed into uniform and thin films.
Under the situation, Japanese Patent Laid-Open No. 210790/1990 discloses an organic EL device in which the light-emitting layer comprises an organic compound having both hole-transporting capabilities and electron-transporting capabilities. However, the device disclosed is still problematic in that the organic compounds applicable thereto are limited for the combination of their structures for electron transportation and hole transportation. Specifically, it is difficult to well control the compounds for their electron-transporting capabilities and hole-transporting capabilities, and, in addition, the balance of those transporting capabilities of the compounds is not good.
What is more, the electron-transporting capabilities and the hole-transporting capabilities of the organic compounds usable in the device disclosed are still unsatisfactory. Therefore, the organic EL device with the compound of that type could hardly achieve high luminance. In addition, the compounds are poor in heat resistance and durability. Therefore, still another problem with the device is that, when it is driven continuously, the compound therein is readily crystallized with the result that the luminance of the device is often lowered.
The present inventors have found that, when a compound having by itself both an electron-transporting unit with good electron-transporting capabilities and a hole-transporting unit with good hole-transporting capabilities in the molecule, in which the units are bonded to each other directly or via a linking group, is present in a light-emitting layer in an organic EL device, it could exhibit well-balanced electron-transporting capabilities and hole-transporting capabilities, that the light-emitting layer containing the compound of that type is highly durable even though it has a single-layered structure, and that the organic EL device comprising the light-emitting layer of that type produces high luminance, and have completed the present invention.
The properties of the hole-transporting unit in the compound are characterized in that the unit exhibits by itself good hole-transporting capabilities and can be formed into thin and durable films with no problem. Similarly, the properties of the electron-transporting unit in the compound are characterized in that the unit exhibits by itself good electron-transporting capabilities and can be formed into thin and durable films with no problem.
Specifically, the object of the invention is to provide an organic EL device which produces high luminance and is durable and which can be fabricated with ease.
The invention relates to an organic EL device comprising two electrodes and a light-emitting layer sandwiched therebetween, in which the light-emitting layer comprises a molecular compound having an electron-transporting unit and a hole-transporting unit bonded to each other directly or via a linking group.
With the molecular compound of that type having an electron-transporting unit and a hole-transporting unit bonded to each other directly or via a linking group, the electron-transporting capabilities and the hole-transporting capabilities of the light-emitting layer are well balanced in the organic EL device. Having the light-emitting layer of that type, therefore, the organic EL device produces high luminance and is durable.
In the organic EL device of the invention, it is preferable that the light-emitting layer contains a recombination site-forming substance.
Around the center of the light-emitting layer thus containing a recombination site-forming substance, electrons and holes can be recombined. Having the light-emitting layer of the preferred type, therefore, the organic EL device produces higher luminance.
In the organic EL device of the invention, it is also preferable that the electron mobility through the electron-transporting unit is at least 1xc3x9710xe2x88x926 cm2/Vs. Similarly in the device, it is also preferable that the hole mobility through the hole-transporting unit is at least 1xc3x9710xe2x88x926 cm2/Vs. The charge mobility through the unit can be determined, for example, by forming the unit into a thin film, put the film between a pair of electrodes to fabricate a device, and measuring the device for the charge mobility through the unit therein according to a method of time of flight or the like.
In the organic EL device of the invention, it is also preferable that the electron-transporting unit is of at least one compound selected from perinone derivatives, oxadiazole derivatives, triazole derivatives, phenanthroline derivatives, quinoxaline derivatives, sirol derivatives, condensed polycyclic aromatic derivatives having at least three rings, and quinoline complex derivatives.
In the organic EL device of the invention, it is also preferable that the hole-transporting unit is of a combination of triphenylamines and stilbene derivatives, or of any one of the compounds.
In the organic EL device of the invention, it is also preferable that the linking group is at least one selected from alkylene groups, vinylene groups, ethanediylidene groups, styryl groups, ether groups, amine groups and arylene groups. The linking group may form a part of the electron-transporting unit or the hole-transporting unit.
In the organic EL device of the invention, it is more preferable that the molecular compound is at least one selected from those of the following formulae (1) to (14). In formula (4), R1, R2, R3 and R4 may be the same or different and each represents an alkyl group having from 1 to 10 carbon atoms, an aryl group having from 6 to 20 carbon atoms, an alkyl-substituted aryl group having from 6 to 20 carbon atoms, or an aryloxyaryl group having from 12 to 30 carbon atoms. 
In the organic EL device of the invention, it is still preferable that the recombination site-forming substance is at least one compound selected from styrylamine compounds, quinacridone derivatives, rubrene derivatives, coumarin derivatives and pyran derivatives.
In the organic EL device of the invention, it is further preferable that the electron-transporting unit and the hole-transporting unit do not form an exciplex or a charge transfer complex. If the units form an exciplex or a charge transfer complex, the exciplex or the charge transfer complex formed in the light-emitting layer will interfere with energy transfer to the recombination site-forming substance in the layer, whereby the luminance of the device and even the luminous efficiency thereof will be much lowered.
In the organic EL device of the invention, the light-emitting layer may be of a copolymer having an electron-transporting unit and a hole-transporting unit. Preferably, the copolymer has the electron-transporting unit and the hole-transporting unit both as repetitive units therein. Especially preferably the copolymer a structure represented by:
[(hole-transporting unit)mxe2x88x92(electron-transporting unit)n]xxe2x88x92, 
wherein m greater than 1, n greater than 1, and x greater than 1.
Concretely, for example, usable are copolymers having a structure of a diamine-type hole-transporting unit combined with an oxadiazole-type electron-transporting unit, which is represented by the following formula (15): 
and copolymers having a structure of a diamine-type hole-transporting unit combined with a quinoline complex-type electron-transporting unit, which is represented by the following formula (16): 
Needless-to-say, these are not limitative, and other various compounds in a broad range are usable herein. However, linking groups from ethers, esters, ketones and the like are unfavorable, as causing electron trapping and therefore often degrading the light-emitting layer in the organic EL device.